Check out this link for slick ways to make air core inductors...
http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=12823
I'm going to wind one out of 1/4" copper tubing as soon as I figure out the turns and diameter needed.
Regards,Lee
http://www.freewebs.com/wilcomlabs/index.htm
Check out this link for slick ways to make air core inductors...
http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=12823
I'm going to wind one out of 1/4" copper tubing as soon as I figure out the turns and diameter needed.
Regards,Lee
http://www.freewebs.com/wilcomlabs/index.htm
"Thanks much for your willingness to share your adventures and for the great writeups with pics. Some of the best technical advances happen when investigators share the step by step investigations during development rather than waiting until everything is perfect and polished. For example, anyone wanting to follow your lead knows the small wire is not optimum and can save $$$ and time. You also are getting feedback which we all hope is useful.
So far as feedback, it's always a mixed bag. It's good to read it and learn from all the assorted sites and measurements done under as close to lab conditions as possible, so long as it doesn't stifle your own spirit of "try it and see" in the non-optimal real world.
I'm fairly sure that most of the conditions mentioned for measuring the Q of *very* high Q coils would be nullified to at least some degree in mounting it as part of a physical antenna. Particularly so with an indoor antenna where even if we discount how the parts of the rest of the antenna would damp the Q, there are likely to be objects like walls made of diverse building materials and movable parameters like people walking near to it.
My interest in the spider coil was initially that it is physically not as large as the solenoid coil. With an indoor antenna, in many cases people's houses don't have 3 meter high ceilings. So if a person had to make do with say 6 ft of vertical radiator (allowing a foot or two for some sort of stand for the antenna system and also a bit for the wire to ground) , taking a foot or so of that height for a solenoid coil would have a more significant impact on how much radiator is left to work with than a spider web coil that might take up an inch or so of the available space. The general concensus has seemed to be that loading coils do not generally radiate and so it seems most people don't count them in the height with an outdoor antenna, but with an indoor antenna the ceiling simply may not be that high.
One of the first experiments (albeit a crude one) that I did with the spider web coils I built was to bring metal objects near while watching the inductance readout on my meter. I noted that non-ferrous metal (a roll of aluminum foil from the kitchen cupboard) moved the inductance down when brought near, while ferrous material (the crescent wrench seen nearby in one of the photos) moved it up. Which was as expected. BUT.. The shift in both cases was only pronounced when the object was brought near the "hole" in the center of the coil.. there it started to shift uH when a few inches away.. Bringing the metal object up to the edge of the coil didn't shift the inductance even 1 uH until I was within a couple wire diameters. Whether the object was held in fingers or hung from a bit of twine didn't seem to make a noticeable difference.
I haven't wound a similar solenoid coil to compare how it "picks up" the presence of objects compared to the spider web coil, but my current logic based on past experience with the small spaced-turn air core coils used at FM frequencies is that they are very sensitive to the presence of a hand near the winds. Might be more a VHF thing, might not.. But that's what experiments are for. If the spider web coil is better than a solenoid coil for not shifting inductance (or maybe other values, since I'm assuming the Q is being affected long before a shift in inductance of the degree I was looking for is observed) when objects are near or pass near, that might be another advantage to consider for indoor antennas.
Also I picked the spider web because it is fairly easy and inexpensive to construct as an air-core coil. With the typical solenoid coils used with the sort of outdoor antenna usually made for the SStran, I've wondered how much the tuning is being affected by moisture/dampness. A formless air core coil would dry quicker after rain or condensation and as such might be more consistent. I'd say the 22 gauge "coil 2" I made "might" be physically robust enough for outdoor mounting. But a heavier gauge like 18 or 16 would probably hold up pretty well outdoors. The current project is an indoor antenna experiment, but I hope that some of it is also applicable to an outdoor antenna this summer.
I learn best by tinkering. For example, all the things I'd read about "stub tuning" with a piece of coax made a lot more sense after I got a cheap capacitance/inductance/resistance meter and nipped some bits off some old rg 59. And slipping a piece of coax into a piece of brass tubing that was a fairly tight fit and tinkering with the coax's braid and the tube hooked in parallel to the tuning cap of a cheap transistor radio made the concept of a "trombone" capacitor considerably clearer for me and was also quite entertaining. (Ok, so maybe I'm easilly entertained. LOL)
Projects that can be tinkered together have a definite place in the hobby if we want to encourage learning and more (and/or better) part15 stations. I have *some* tech background, but far less than some folks here. Many people that might look into part15 have more of an interest in dj-ing/programming than in engineering. But I'm pretty sure most of them could manage at the very least putting together a salt-box or oatmeal-box crystal radio receiver from clear instructions if that was the *only* way to get an affordable receiver to listen to radio on. That's kind of the situation they're in with part15 antennas. They could order an outdoor one from Carl the antenna guy maybe.. But for indoor, I didn't find much of anything out there on the net at all. Not for sale *or* for homebrewing. Summer is a ways away yet here in the northern part of the US and for some people who may have an interest in the hobby, even when it gets here they might not have a yard/situation that allows for an outdoor antenna anyway. If they don't have a yard or they're in a bad neighborhood, an indoor antenna may be their only option.
Now while it does take a bit of soldering ability to homebrew or build a kit, a person can solder well enough (or teach themselves or find someone to teach them) if they want to get on the air badly enough, but still know nothing at all about antennas. "Optimal" may not be in the reach of everyone, but "better" almost always can be. Especially when compared with a simple short wire indoor antenna. A lot of people learn best a bit at a time as they go along, and even "a bit better" can be a powerful and fun motivator. If we want more people in the hobby, those are people we have to reach. And the easiest way I can think of are easy inexpensive projects they can use with their stations.
Crystal radios have been making a comeback, from all I can tell. I've looked at a lot of crystal radio sites and forums, especially lately.. And you know the one thing I don't think I've ever seen on even one of them? I don't think I've seen *anybody* telling a newbie to "not bother" with making an oatmeal box radio if they can get the parts easily. Oh, yeah, some of the sites and discussions are very tech heavy and some of the advanced DX designs could scare off any newbie. But the overall tone of that community seems encouraging to "hands on" and even very simple projects to learn solid basics if that's what it takes to get people to learn and do. We need that for part15 if we want to keep enough people involved to keep the hobby alive and growing in good ways.
It goes without saying that we also stress and reinforce the legal requirements and rules and responsibilities, since transmitting stations (even very small ones) need to have those priorities where someone with a crystal receiver does not. We do plenty of that here, to the point where a newbie might find it redundant or tiresome. Heck, I think *all* of us find it tiresome at least once in a while. LOL But it is a necessary part encouraging *responsible* operation of a part15 transmitter/station. Better to say it too many times than to say it too few and have someone unknowingly put themselves at risk for fines and etc.
So part of my indoor antenna project will be taking into account the 3 meters total for the antenna, feedline and ground wire. May not work everywhere or for every situation, but at least *one* way it can be done. I'll get to that as I figure out a practical way of doing it with easily available materials, though.
I think I've monopolized the soapbox for long enough just now, though.. LOL
Daniel
"Thanks much for your willingness to share your adventures and for the great writeups with pics. Some of the best technical advances happen when investigators share the step by step investigations during development rather than waiting until everything is perfect and polished. For example, anyone wanting to follow your lead knows the small wire is not optimum and can save $$$ and time. You also are getting feedback which we all hope is useful.
So far as feedback, it's always a mixed bag. It's good to read it and learn from all the assorted sites and measurements done under as close to lab conditions as possible, so long as it doesn't stifle your own spirit of "try it and see" in the non-optimal real world.
I'm fairly sure that most of the conditions mentioned for measuring the Q of *very* high Q coils would be nullified to at least some degree in mounting it as part of a physical antenna. Particularly so with an indoor antenna where even if we discount how the parts of the rest of the antenna would damp the Q, there are likely to be objects like walls made of diverse building materials and movable parameters like people walking near to it.
My interest in the spider coil was initially that it is physically not as large as the solenoid coil. With an indoor antenna, in many cases people's houses don't have 3 meter high ceilings. So if a person had to make do with say 6 ft of vertical radiator (allowing a foot or two for some sort of stand for the antenna system and also a bit for the wire to ground) , taking a foot or so of that height for a solenoid coil would have a more significant impact on how much radiator is left to work with than a spider web coil that might take up an inch or so of the available space. The general concensus has seemed to be that loading coils do not generally radiate and so it seems most people don't count them in the height with an outdoor antenna, but with an indoor antenna the ceiling simply may not be that high.
One of the first experiments (albeit a crude one) that I did with the spider web coils I built was to bring metal objects near while watching the inductance readout on my meter. I noted that non-ferrous metal (a roll of aluminum foil from the kitchen cupboard) moved the inductance down when brought near, while ferrous material (the crescent wrench seen nearby in one of the photos) moved it up. Which was as expected. BUT.. The shift in both cases was only pronounced when the object was brought near the "hole" in the center of the coil.. there it started to shift uH when a few inches away.. Bringing the metal object up to the edge of the coil didn't shift the inductance even 1 uH until I was within a couple wire diameters. Whether the object was held in fingers or hung from a bit of twine didn't seem to make a noticeable difference.
I haven't wound a similar solenoid coil to compare how it "picks up" the presence of objects compared to the spider web coil, but my current logic based on past experience with the small spaced-turn air core coils used at FM frequencies is that they are very sensitive to the presence of a hand near the winds. Might be more a VHF thing, might not.. But that's what experiments are for. If the spider web coil is better than a solenoid coil for not shifting inductance (or maybe other values, since I'm assuming the Q is being affected long before a shift in inductance of the degree I was looking for is observed) when objects are near or pass near, that might be another advantage to consider for indoor antennas.
Also I picked the spider web because it is fairly easy and inexpensive to construct as an air-core coil. With the typical solenoid coils used with the sort of outdoor antenna usually made for the SStran, I've wondered how much the tuning is being affected by moisture/dampness. A formless air core coil would dry quicker after rain or condensation and as such might be more consistent. I'd say the 22 gauge "coil 2" I made "might" be physically robust enough for outdoor mounting. But a heavier gauge like 18 or 16 would probably hold up pretty well outdoors. The current project is an indoor antenna experiment, but I hope that some of it is also applicable to an outdoor antenna this summer.
I learn best by tinkering. For example, all the things I'd read about "stub tuning" with a piece of coax made a lot more sense after I got a cheap capacitance/inductance/resistance meter and nipped some bits off some old rg 59. And slipping a piece of coax into a piece of brass tubing that was a fairly tight fit and tinkering with the coax's braid and the tube hooked in parallel to the tuning cap of a cheap transistor radio made the concept of a "trombone" capacitor considerably clearer for me and was also quite entertaining. (Ok, so maybe I'm easilly entertained. LOL)
Projects that can be tinkered together have a definite place in the hobby if we want to encourage learning and more (and/or better) part15 stations. I have *some* tech background, but far less than some folks here. Many people that might look into part15 have more of an interest in dj-ing/programming than in engineering. But I'm pretty sure most of them could manage at the very least putting together a salt-box or oatmeal-box crystal radio receiver from clear instructions if that was the *only* way to get an affordable receiver to listen to radio on. That's kind of the situation they're in with part15 antennas. They could order an outdoor one from Carl the antenna guy maybe.. But for indoor, I didn't find much of anything out there on the net at all. Not for sale *or* for homebrewing. Summer is a ways away yet here in the northern part of the US and for some people who may have an interest in the hobby, even when it gets here they might not have a yard/situation that allows for an outdoor antenna anyway. If they don't have a yard or they're in a bad neighborhood, an indoor antenna may be their only option.
Now while it does take a bit of soldering ability to homebrew or build a kit, a person can solder well enough (or teach themselves or find someone to teach them) if they want to get on the air badly enough, but still know nothing at all about antennas. "Optimal" may not be in the reach of everyone, but "better" almost always can be. Especially when compared with a simple short wire indoor antenna. A lot of people learn best a bit at a time as they go along, and even "a bit better" can be a powerful and fun motivator. If we want more people in the hobby, those are people we have to reach. And the easiest way I can think of are easy inexpensive projects they can use with their stations.
Crystal radios have been making a comeback, from all I can tell. I've looked at a lot of crystal radio sites and forums, especially lately.. And you know the one thing I don't think I've ever seen on even one of them? I don't think I've seen *anybody* telling a newbie to "not bother" with making an oatmeal box radio if they can get the parts easily. Oh, yeah, some of the sites and discussions are very tech heavy and some of the advanced DX designs could scare off any newbie. But the overall tone of that community seems encouraging to "hands on" and even very simple projects to learn solid basics if that's what it takes to get people to learn and do. We need that for part15 if we want to keep enough people involved to keep the hobby alive and growing in good ways.
It goes without saying that we also stress and reinforce the legal requirements and rules and responsibilities, since transmitting stations (even very small ones) need to have those priorities where someone with a crystal receiver does not. We do plenty of that here, to the point where a newbie might find it redundant or tiresome. Heck, I think *all* of us find it tiresome at least once in a while. LOL But it is a necessary part encouraging *responsible* operation of a part15 transmitter/station. Better to say it too many times than to say it too few and have someone unknowingly put themselves at risk for fines and etc.
So part of my indoor antenna project will be taking into account the 3 meters total for the antenna, feedline and ground wire. May not work everywhere or for every situation, but at least *one* way it can be done. I'll get to that as I figure out a practical way of doing it with easily available materials, though.
I think I've monopolized the soapbox for long enough just now, though.. LOL
Daniel
Daniel,
You wrote"
...So far as feedback, it's always a mixed bag...."
I understand and I agree. Several of my design projects were messed up by managers insisting on features that the customers did not ask for nor need. One I recall went from a simple $15 at Radio Shack for parts product (sell for $100 to a customer itching to buy) to a $500 our cost (sell for $1000 ...too low a margin) product that the customer wouldn't buy due to cost. My boss insisted on a bunch of bells and whistles that weren't needed and delayed delivery and increased cost and this killed the product.
The other problem is a deluge of advice can delay the design to the point that it never is finished. I have lived this both as a designer and a project manager.
As hobbyists, we have an advantage in that we are the bosses as well as the customers and have total control of the project. We are free to use or not use the advice offered. I learn from advice but I no longer have to let it dictate my progress.
With that being said, advice is always welcomed by me but I will not let it interfere with my projects, only enhance them. I think you have figured this out and you are wise to forge ahead as you planned.
(Nice talk Russ...).
Neil
Daniel,
You wrote"
...So far as feedback, it's always a mixed bag...."
I understand and I agree. Several of my design projects were messed up by managers insisting on features that the customers did not ask for nor need. One I recall went from a simple $15 at Radio Shack for parts product (sell for $100 to a customer itching to buy) to a $500 our cost (sell for $1000 ...too low a margin) product that the customer wouldn't buy due to cost. My boss insisted on a bunch of bells and whistles that weren't needed and delayed delivery and increased cost and this killed the product.
The other problem is a deluge of advice can delay the design to the point that it never is finished. I have lived this both as a designer and a project manager.
As hobbyists, we have an advantage in that we are the bosses as well as the customers and have total control of the project. We are free to use or not use the advice offered. I learn from advice but I no longer have to let it dictate my progress.
With that being said, advice is always welcomed by me but I will not let it interfere with my projects, only enhance them. I think you have figured this out and you are wise to forge ahead as you planned.
(Nice talk Russ...).
Neil
I was particularly interested in Item 6 in Macrohenry's post, which dealt with Q measurement using Ben Tongue's method. Ben Tongue's method has been discussed recently in another thread. To reiterate, the method consists of applying a small amount of RF power from a signal generator to a small coil. The signal from this small coil is loosely coupled to the cold end of the coil under test. The coil under test is connected to an air-variable tuning capacitor, and the coil and capacitor are tuned to resonance at the desired test frequency. The ground clip lead of a 1X oscilloscope probe is connected to the cold end of the coil under test. The probe tip is clipped to the hot end of the coil under test, but a direct conductive connection is not made. The probe tip is clipped to an insulated wire at the hot end of the coil under test. This provides for very low capacitive coupling between the hot end of the coil and and the 1X oscilloscope probe. The signal generator either has a digital frequency readout, or it is connected to a frequency counter. Initially, the signal generator frequency is adjusted for a peak reading on the oscilloscope, and the amplitude at resonance is noted. Then the bandwidth between the 3 dB points (1/Sqrt(2) below the amplitude at resonace) is found. This bandwidth is divided into the resonant frequency to give the Q.
Macrohenry states that Ben Tongue's method does not accurately measure the Q of a coil if the Q is very high. Macrohenry recommends using an FET buffer amplifier at the hot end of the coil under test to get more accurate Q measurements. I did not use the circuit Macrohenry recommended, but I have another similar circuit available. It consists of a 2N4416 source follower with a 727 ohm source resistor to ground. I used this circuit, which is constructed on a small copper-clad fiberglass board, to isolate the hot end of the coil from a 10X oscilloscope probe. Based solely on circuit analysis, I determined that the input impedance of the buffer is about 30 M ohms in parallel with about 3 pF. This input impedanci is enough to significantly reduce the Q reading of a loading coil. For a particular coil that I had tested using Ben Tongue's method, I obtained a Q of 438. Using the FET buffer reduced the Q reading to 349. The FET buffer just applied too much of a load to the coil under test. This additional load was not present when using Ben Tongue's method.
There is a counterintuitive aspect of Ben's method that I wish to point out. The resistance in series with the signal coil needs to be higher than one would expect. I originally used a small scramble-wound signal coil, with an inductance of 67.3 uH, with a 27 ohm resistor between the signal generator and the coil. Increasing the resistance to 820 ohms resulted in considerably higher Q readings. It at first seemed to me that the the higher resistance in series with the signal coil would reduce the Q of the test system, but the opposite turned out to be the case. I verified this conclusion by performing a circuit analysis of the loosely-coupled RF transformer that constitutes the test setup. The signal coil should be located as far from the coil under test as possible in order to maximize Q readings. The mutual inductance must be low to get accurate results. I use so much separation between coils that I see appreciable 60 Hz interference. This is not a problem when the oscilloscope is used with line sync.
Macrohenry mentioned obtaining poor results in Q measurements when using a Heathkit oscilloscope. I have never used a Heathkit oscilloscope, but I have used a Heathkit PK-1 probe with an EICO Model 460 oscilloscope many years ago. After I upgraded to a Telequipment D52 oscilloscope (which I still have, but only as a backup for my "good" scope), I modified my Heathkit scope probe from banana plugs to a coaxial connector, and continued to use it. I still have the probe, and I tried to obtan a Q measurement with it. I got a Q reading of 380 using the Heathkit probe compared to 438 when I used an old Textronix 1X probe. A slight lowering of the resonant frequency I obtained indicated that using the Heathkit probe added about 2 pF to the test circuit. Also, a higher signal on the oscilloscope indicated that there was more capacitive coupling from the hot end of the coil under test when using the Heathkit probe than with the Tektronix probe, causing the Heathkit probe to function as a heavier load than the Tektronix probe. The Heathkit probe does not have a clip at the tip (just a sharp point). I tied a loop of insulated wire around the tip to hold the probe in place.
Ben Tongue recommends measuring Q over a sheet of metal used as a ground plane. This practice reduces the Q measurement, as Macrohenry's recommendation of keeping the coil under test away from metal would suggest. I, myself, perform Q measurements keeping the coil under test away from metal, but I can see why Ben recommends testing above a ground plane. Testing above a ground plane keeps the Q measurement more consistent. Rattan explained the situation very well in this insightful comment in his last post in this thread.
"I'm pretty sure that most of the conditions for measuring the Q of very high Q coils will be nullified to some degree in mounting it as part of a physical antenna."
Using a metallic ground plane when measuring the unloaded Q of a coil measures said unloaded Q under more realistic conditions. The self-capacitance of the coil, which affects Q a great deal, does not depend only upon the the properties of the isolated coil itself, but upon its surroundings. For the coil I measured for this post, I used a dip meter to measure the self-resonant frequency of the isolated coil. The self-capacitancce of the coil measured to be only .76 pF. When measuring the Q using Ben Tongue's method, the self-capacitance measured to be 11.3 pF. I was not using a ground plane, but the cold end of the coil was connected to ground through the ground lead of the oscilloscope probe. When using a ground plane, the self-capacitance of the coil increases, depending upon how far above the ground plane the coil is located. More consistency in results can be obtained if a consensus can be reached about a standard height (like a foot, for example) above a ground plane at which a Q test should be performed.
Because of the variables that are involved, Q is, at least partially, in the eye of the beholder. Different people get different Q measurements while testing the same coil. Some people consistently get high Q, and others consistently get low Q. Wishful thinking plays a part in Q measurement. For high-Q coils, the tests are not very robust because the measurements are at the threshold of detectability of the test instrumentation. It is not easy to get the exact resonant frequency of the coil because the peak of the resonance curve is nearly flat. There is a considerable region near resonance where the amplitude seen on the oscilloscope does not change much. Similarly, it is not easy to detect the 3 dB points because the resonance curve is quite smooth at these points. This is where wishful thinking becomes a significant part of measurement error. If the tester is completely objective, the average of many tests would give an accurate Q measurement. But testers, being people, are likely to (perhaps unconsciously) bias the tests.
I noticed that several Q measurements on the web sites linked by Macrohenry report measurements above 1000 using the HP (Agilent) 4342A Q meter, even though the upper limit of this meter is 1000. In principle, higher Qs can be measured by adding a known loss resistance to the coil under test. This practice can result in unsatisfactory results. Adding loss resistance broadens the resonance peak, making the resulting corrected Q measurement inaccurate.
I was particularly interested in Item 6 in Macrohenry's post, which dealt with Q measurement using Ben Tongue's method. Ben Tongue's method has been discussed recently in another thread. To reiterate, the method consists of applying a small amount of RF power from a signal generator to a small coil. The signal from this small coil is loosely coupled to the cold end of the coil under test. The coil under test is connected to an air-variable tuning capacitor, and the coil and capacitor are tuned to resonance at the desired test frequency. The ground clip lead of a 1X oscilloscope probe is connected to the cold end of the coil under test. The probe tip is clipped to the hot end of the coil under test, but a direct conductive connection is not made. The probe tip is clipped to an insulated wire at the hot end of the coil under test. This provides for very low capacitive coupling between the hot end of the coil and and the 1X oscilloscope probe. The signal generator either has a digital frequency readout, or it is connected to a frequency counter. Initially, the signal generator frequency is adjusted for a peak reading on the oscilloscope, and the amplitude at resonance is noted. Then the bandwidth between the 3 dB points (1/Sqrt(2) below the amplitude at resonace) is found. This bandwidth is divided into the resonant frequency to give the Q.
Macrohenry states that Ben Tongue's method does not accurately measure the Q of a coil if the Q is very high. Macrohenry recommends using an FET buffer amplifier at the hot end of the coil under test to get more accurate Q measurements. I did not use the circuit Macrohenry recommended, but I have another similar circuit available. It consists of a 2N4416 source follower with a 727 ohm source resistor to ground. I used this circuit, which is constructed on a small copper-clad fiberglass board, to isolate the hot end of the coil from a 10X oscilloscope probe. Based solely on circuit analysis, I determined that the input impedance of the buffer is about 30 M ohms in parallel with about 3 pF. This input impedanci is enough to significantly reduce the Q reading of a loading coil. For a particular coil that I had tested using Ben Tongue's method, I obtained a Q of 438. Using the FET buffer reduced the Q reading to 349. The FET buffer just applied too much of a load to the coil under test. This additional load was not present when using Ben Tongue's method.
There is a counterintuitive aspect of Ben's method that I wish to point out. The resistance in series with the signal coil needs to be higher than one would expect. I originally used a small scramble-wound signal coil, with an inductance of 67.3 uH, with a 27 ohm resistor between the signal generator and the coil. Increasing the resistance to 820 ohms resulted in considerably higher Q readings. It at first seemed to me that the the higher resistance in series with the signal coil would reduce the Q of the test system, but the opposite turned out to be the case. I verified this conclusion by performing a circuit analysis of the loosely-coupled RF transformer that constitutes the test setup. The signal coil should be located as far from the coil under test as possible in order to maximize Q readings. The mutual inductance must be low to get accurate results. I use so much separation between coils that I see appreciable 60 Hz interference. This is not a problem when the oscilloscope is used with line sync.
Macrohenry mentioned obtaining poor results in Q measurements when using a Heathkit oscilloscope. I have never used a Heathkit oscilloscope, but I have used a Heathkit PK-1 probe with an EICO Model 460 oscilloscope many years ago. After I upgraded to a Telequipment D52 oscilloscope (which I still have, but only as a backup for my "good" scope), I modified my Heathkit scope probe from banana plugs to a coaxial connector, and continued to use it. I still have the probe, and I tried to obtan a Q measurement with it. I got a Q reading of 380 using the Heathkit probe compared to 438 when I used an old Textronix 1X probe. A slight lowering of the resonant frequency I obtained indicated that using the Heathkit probe added about 2 pF to the test circuit. Also, a higher signal on the oscilloscope indicated that there was more capacitive coupling from the hot end of the coil under test when using the Heathkit probe than with the Tektronix probe, causing the Heathkit probe to function as a heavier load than the Tektronix probe. The Heathkit probe does not have a clip at the tip (just a sharp point). I tied a loop of insulated wire around the tip to hold the probe in place.
Ben Tongue recommends measuring Q over a sheet of metal used as a ground plane. This practice reduces the Q measurement, as Macrohenry's recommendation of keeping the coil under test away from metal would suggest. I, myself, perform Q measurements keeping the coil under test away from metal, but I can see why Ben recommends testing above a ground plane. Testing above a ground plane keeps the Q measurement more consistent. Rattan explained the situation very well in this insightful comment in his last post in this thread.
"I'm pretty sure that most of the conditions for measuring the Q of very high Q coils will be nullified to some degree in mounting it as part of a physical antenna."
Using a metallic ground plane when measuring the unloaded Q of a coil measures said unloaded Q under more realistic conditions. The self-capacitance of the coil, which affects Q a great deal, does not depend only upon the the properties of the isolated coil itself, but upon its surroundings. For the coil I measured for this post, I used a dip meter to measure the self-resonant frequency of the isolated coil. The self-capacitancce of the coil measured to be only .76 pF. When measuring the Q using Ben Tongue's method, the self-capacitance measured to be 11.3 pF. I was not using a ground plane, but the cold end of the coil was connected to ground through the ground lead of the oscilloscope probe. When using a ground plane, the self-capacitance of the coil increases, depending upon how far above the ground plane the coil is located. More consistency in results can be obtained if a consensus can be reached about a standard height (like a foot, for example) above a ground plane at which a Q test should be performed.
Because of the variables that are involved, Q is, at least partially, in the eye of the beholder. Different people get different Q measurements while testing the same coil. Some people consistently get high Q, and others consistently get low Q. Wishful thinking plays a part in Q measurement. For high-Q coils, the tests are not very robust because the measurements are at the threshold of detectability of the test instrumentation. It is not easy to get the exact resonant frequency of the coil because the peak of the resonance curve is nearly flat. There is a considerable region near resonance where the amplitude seen on the oscilloscope does not change much. Similarly, it is not easy to detect the 3 dB points because the resonance curve is quite smooth at these points. This is where wishful thinking becomes a significant part of measurement error. If the tester is completely objective, the average of many tests would give an accurate Q measurement. But testers, being people, are likely to (perhaps unconsciously) bias the tests.
I noticed that several Q measurements on the web sites linked by Macrohenry report measurements above 1000 using the HP (Agilent) 4342A Q meter, even though the upper limit of this meter is 1000. In principle, higher Qs can be measured by adding a known loss resistance to the coil under test. This practice can result in unsatisfactory results. Adding loss resistance broadens the resonance peak, making the resulting corrected Q measurement inaccurate.